γ-Al2O3 thin film formation via oxidation of β-NiAl(1 1 0) Original Research Article

β-NiAl(1 1 0) was oxidized in air for 1–2 h in the temperature range of 650–950 °C. The structure and morphology of the oxide films were characterized using a cross-sectional transmission electron microscopy (TEM) method. Only a thin film of aluminum oxide, γ-Al2O3, was formed. The epitaxial relationship between NiAl and γ-Al2O3 as well as the surface roughness depends on the oxidation temperature. The Nishiyama–Wassermann (NW) orientation relation (OR) between β-NiAl and γ-Al2O3 was noted at an oxidation temperature of 850 °C while the Kurdjumov–Sachs (KS) OR was observed at 650 °C. The changes in the epitaxial relationship between the γ-Al2O3 film and the NiAl substrate were caused by lattice mismatch-induced strain energy during oxide growth. It was also noted that short time oxidation at T = 750 °C created γ′ phase precipitates between the NiAl substrate and the γ-Al2O3 film, while oxidation at the higher temperature of 950 °C resulted in textured polycrystalline γ-Al2O3 films. The smoothest single crystal epitaxial γ-Al2O3 film formed at an oxidation temperature of 850 °C in air.